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Tightly coupled mitochondria from M . longissimus dorsi (LD) muscle of different breeds of pigs showed variations in the rate of Ca2+ efflux during anaerobiosis. The initial fast efflux rate of Ca2 + could be used to relate the stress-susceptibility of the pigs used. Mitochondria from LD muscle of stress-susceptible (SS) Pietrain and Poland China pigs showed an anaerobic rate of Ca2 + efflux twice that of stress-resistant (SR) Large Whites. Mitochondria from crossbreds (Large White x Saddleback) showed a Ca2 + efflux rate intermediate between that of purebred SS and SR pigs. Halothane enhanced the rate of Ca2 + efflux only in SS pigs, and this effect could be eliminated by MgC12. The marked difference in the rate of CaZ + efflux between SS and SR pigs, and the enhancement in the rate of Ca2 + efflux by halothane only in SS pigs, offers an ultimate explanation for the formation of PSE (pale, soft and exudative) muscle and malignant hyperthermia in SS pigs. The excess Ca2 + liberated from mitochondria of SS pigs is postulated to be the trigger for PSE meat and also the case of malignant h yperthermia.

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